#include <fstream>
#include <iomanip>
#include <cmath>

int main() {
	std::ifstream fin("in.txt");
	std::ofstream fout("out.txt");

	double ans = 0;

	double px, py;
	double fx, fy;
	bool firstly = true;

	while (1) {
		double x, y;
		fin >> x >> y;
		if (fin.fail()) {
			break;
		}
		
		if (firstly) {
			firstly = false;
			fx = x;
			fy = y;
		} else {
			ans += x * py - y * px;			
		}
		px = x;
		py = y;
	}

	ans += fx * py - fy * px;

	fout << std::setprecision(18) << abs(ans/2);	

	return 0;
}